This invention relates to a method and to apparatus for controlling a centrifugal compressor. More particularly the herein invention concerns measuring various operating parameters and controlling inlet guide vanes and/or diffuser vanes to effectively regulate power consumption while preventing operation under surge conditions.
Centrifugal compressors are used in many process applications and can be used in any application where it is desirable to increase the pressure of large volumes of gaseous material. Centrifugal compressors are typically powered by an electric motor at a preselected operating speed. The electrical energy consumed by the motor to drive the compressor varies with the work done by the compressor which is a function of the volume flow of gas therethrough. The volume flow rate of gas entering the compressor is controlled by inlet guide vanes which are positioned to regulate the flow of gas into compressor inlet.
In order to efficiently operate a compressor it is important to control the inlet guide vanes in response to a system parameter such that the energy consumption of the motor driving the compressor may be minimized. The parameter to which the inlet guide vanes can be controlled may include pressure or the volume of the discharge from the compressor. By effectively matching the volume flow through the compressor to the load on the system the power consumed by the motor may be minimized.
A centrifugal compressor under certain operating conditions will enter surge. Surge may occur in the portion of the operating range of the compressor where the volume flow and pressure are such that the flow through the compressor diffuser is aerodynamically unstable and may actually flow backwards. Operating a compressor under these conditions creates severe noise, high mechanical stresses and may result in immediate failure of the compressor. It is desirable to avoid operating under these conditions hence minimum pressure differentials occasioned by minimum flow requirements and the necessary power consumption to the motor to achieve these minimum power requirements are necessary to avoid operating in surge.
A centrifugal compressor operates by receiving gas and accelerating that gas outwardly along an impeller. The gas is then discharged into the diffuser which extends radially outward from the impeller. Within the diffuser velocity pressure generated by the impeller is converted to static pressure to achieve the pressure gain across the compressor. Under full load operating conditions the gas being discharged from the impeller has both a tangential vector perpendicular to the impeller blade and a radially outward vector caused by the flow of additional gas into the impeller. A combination of these vectors acts to force the gas spirally outwardly through the diffuser having a flow path of a known length. The diffuser is designed to achieve the desired static pressure gain over this length flow path to have the compressor operate under designed conditions. When the compressor operates at part load conditions the radially outward factor caused by the flow through the compressor is decreased such that the direction of flow discharged from the impeller becomes more tangential to the impeller blade. Under these circumstances the flow path length through the diffuser is increased and the potential for the velocity pressure to overcome the static resistance along the longer flow path is diminished. When the velocity pressure is unable to overcome the static pressure caused by the longer flow path aerodynamic instabilities occur and the centrifugal compressor does not operate as desired.
Also disclosed in this application is the use of movable diffuser vanes located within the diffuser to aid in routing the gas discharged from the impeller through the diffuser. By repositioning these vanes additional control is obtained over the flow path the discharge gas will take through the diffuser.